A system of the abovementioned type is used, for example, in an internal combustion engine of a motor vehicle. A four-stroke working method comprises here not only the compression of the fuel/air mixture or of the combustion air and the expansion owing to the combustion taking place in the combustion chamber but also the charge-changing process. Within the scope of the charge-changing process, the combustion gases are pushed out via the outlet valves and the combustion chamber is charged with fresh mixture or fresh air via the inlet valves. In four-stroke engines lifting valves are used almost exclusively to control the charge-changing process, said valves undergoing an oscillating reciprocating movement during the operation of the internal combustion engine and in this way carrying out the opening process and closing process of the inlet and outlet openings.
The necessary activation mechanism including the valves is referred to as the valve drive. In this context, the object of the valve drive is to open and close the inlet and outlet openings of the combustion chamber at the correct times, with rapid clearance of the largest possible flow cross sections being aimed at in order to keep the throttling losses in the inflowing and outflowing gas flows low and to ensure that the combustion chamber is charged as fully as possible with fresh mixture and that the combustion gases are pushed out effectively, i.e. completely.
According to the prior art, a valve which can be moved along its longitudinal axis between a valve closed position and a valve open position, in order to clear or shut off an inlet opening or outlet opening of a combustion chamber of the internal combustion engine, is generally used for this purpose. In order to activate the valve, on the one hand valve spring means are provided in order to prestress the valve in the direction of the valve closed position, and on the other hand a valve activation device is used to open the valve counter to the prestressing force of the valve spring means.
The valve activation device comprises a cam shaft on which a plurality of cams is arranged and which is made to rotate by the crank shaft, for example by means of a chain drive, in such a way that the cam shaft, and with it the cams, rotate at half the rotational speed of the crank shaft.
Basically, a distinction is made in this context between a bottom cam shaft and a top cam shaft.
Bottom cam shafts are suitable for activating what are referred to as standing valves, but also with the aid of push rods and levers, for example rocker arms or valve arms for activating suspended valves. Standing valves are opened by pushing them upward, while suspended valves are opened by a downward movement. In this context, a plunger is usually used as an intermediate element, which is in engagement with the cam of the cam shaft, at least during the opening and closing processes.
In contrast, top cam shafts are used exclusively for activating suspended valves, with a valve drive with top cam shaft having a rocker arm, a valve arm or a plunger as a further valve drive component. The rocker arm rotates here about a fixed pivot point and, when deflected by the cam, displaces the valve counter to the prestressing force of the valve spring means in the direction of the valve open position. In the case of a valve arm which can rock about a pivot point which is arranged centrally, the cam acts on one end of the valve arm, with the valve being arranged at the opposite end of the arm.
When a plunger is used, this plunger is fitted on to that end of the rocker valve which is remote from the combustion chamber so that the plunger participates in the oscillating reciprocating movement of the valve when the cam is in engagement, with its cam outer face in the area of the cam lug along a contact line, with the plunger.
An advantage when using top cam shafts is that, in particular as a result of the elimination of the push rod, the moved mass of the valve drive is reduced and the valve drive is more rigid, i.e. less elastic.
Stringent requirements are made of the contour of the cam. On the one hand, the cam is intended, as already mentioned above, to ensure rapid opening and closing of the valves and thus rapid clearance of the largest possible flow cross sections. On the other hand it is necessary to take into account the fact that the valve drive is an elastic mass system which is subjected to severe accelerations and delays owing to the oscillating movement, in particular of the valve and of the plunger. In particular, the cam is to be prevented from lifting off from the plunger at high rotational speeds. A precise mathematical description of the valve drive is very complex and the computational representation of the rotation of the plunger is possible only by estimation. However, at this point a pair of essential aspects which are indispensable for understanding the present invention will be mentioned.
If the cam is in engagement with the plunger, the cam slides with its cam outer face along a contact line on the surface of the plunger. In the process, the rotational movement of the cam results in a reciprocating movement of the plunger. In order to facilitate the sliding and minimize the wear of both components, the contact zone between the cam and plunger is supplied with lubrication oil. Owing to the relative movement of the two components with respect to one another, a lubrication film which has different load-bearing properties depending on the angle of rotation as a result of hydrodynamics is formed between the cam outer face and the surface of the plunger. The structure of this film of lubrication oil is comparable to the structure of the layer of sliding oil in a sliding bearing, while in the present case the lubricity coefficient, which constitutes a measure of the load bearing capacity of the film of lubrication oil is not dependent on the difference between the relative component speeds but rather on the sum of the relative component speeds.
The wear of the cam and plunger is not only disadvantageous in terms of the service life of these components but also in particular in terms of the operational capability of the valve drive. Erosion of material on the cam outer face and/or the plunger surface has, in fact, on the one hand an influence on the valve play and on the other hand effects on the valve stroke and the control times, i.e. on the crank angle, at which the valve is opened and closed.
A further measure for counteracting the wear of the plunger and cam is therefore to arrange the cam and the plunger with respect to one another in such a way that the central plane of the cam which extends perpendicularly with respect to the rotational axis of the cam is arranged offset with respect to the longitudinal axis of the plunger by an eccentricity E1. This eccentricity causes the plunger to rotate about its longitudinal axis when the cam is in engagement, with its cam outer face along a contact line, with the plunger.
The rotation of the plunger is caused by the fact that the areas of the cam outer face located to the left and right of the longitudinal axis of the plunger are of different sizes. The cam areas which are of different sizes act on the plunger—for the most part—with different torque values, for which reason the plunger is made to rotate owing to the difference between these two torque values. The torque values result from the product of the pressure point radius which is manifest as the distance between the respective cam area center from the longitudinal axis of the plunger, and the average frictional force which results from the pressures and the coefficients of friction along the contact line of the cam area in such a way that the average frictional force with the pressure point radius as a lever leads to a torque of a magnitude about the longitudinal axis of the plunger which is equal to the frictional forces which actually occur along the contact line, with their respective levers.
The local pressure along the contact line, and thus also the local lubricity coefficient, i.e. the load bearing capacity in the film of lubrication oil which is formed between the cam outer face and the plunger surface is, as already mentioned above, dependent on the sum of the individual relative component speeds which varies locally at a specific time. This is because the component speed of the plunger varies along the contact line, i.e. it rises as a circumferential speed with increasing radius owing to the rotational movement.
The different local circumferential speeds of the rotating plunger lead in turn to a sum of the relative component speeds which changes along the contact line so that the parameters which are dependent on these variables, in particular the lubricity coefficient, also change along the contact line. If the sum of the relative component speeds is zero, the lubricity coefficient is also zero. If the film of lubrication oil is then no longer supplied with oil, the lubrication film loses load bearing capacity. This is a borderline case such as occurs, for example, when the cam outer face with the critical contact radius is part of the contact line.
A decreasing lubricity coefficient basically has the disadvantage that as the load bearing capacity of the lubrication film decreases, the lubrication film at first increasingly leaves the range of fluid friction and there is a transition to mixed friction, while the proportion of solid body friction increases more and more as the lubricity coefficient decreases further.
Furthermore it is necessary to take into account the fact that the cam outer face of the cam has a radius of curvature which changes locally in the direction of rotation so that the speed with which the cam slides over the plunger surface changes with the rotational angle of the cam at least in the area of the cam lug. This effect also leads to constantly changing conditions in the lubrication film along the contact line. The parameters which are responsible for the load bearing capacity of the lubrication film therefore change firstly locally along the contact line and additionally as a function of time.
In trials, measurements have shown that the rotation of the plunger can vary between the absolute stationary state and, for example, 2000 rpm. This can be explained only by a lubricity coefficient which also changes greatly, i.e. by a load bearing capacity of the film of lubrication oil which changes greatly along the contact line. The continuously changing conditions in the lubrication film along the contact line ultimately also give rise to a fluctuating torque which changes greatly over time about the longitudinal axis of the plunger. This in turn results in a very irregular rotation of the plunger.
From the irregular rotation of the plunger it is therefore possible to draw conclusions about the friction conditions present along the contact line. The very pronounced fluctuations in the rotation of the plunger make it possible to conclude that the friction conditions also change greatly and encompass the entire area from pure fluid friction to solid body friction.
It is possible to assume that there are specific rotational speed ranges for the rotation of a plunger in which there is a relative optimum of the lubrication conditions between the cam and the cam follower element, while partially inadequate lubrication occurs above and below this rotational speed window, resulting in increased mixed friction as a result of contact between solid bodies. Since the wear also increases with an increasing proportion of solid body friction, basically the most wide ranging possible hydrodynamic formation of a lubrication film between the plunger and cam is aimed at.
A basic objective of designers when configuring a valve drive is to keep the wear between the cam and plunger as low as possible.